I wonder what is R/W offset in EAC, why CD read/write offset occurs and how to correct them to make
100% perfect backup.

I found that accuraterip checksums of same track in two nero6-cloned CD differs, also happens in Alcohol120%

How to identify the Offset values and needed correction

For a good explanation I strongly suggest reading the ESSENTIAL Ripping Guide to EAC, click on "offsets" on the left and your questions should be answered.

"What" has been answered many times and also "How". That leaves us "Why", which is a valid question that I have not seen answered yet.

At first sight one would like to think that it is all about manufacturing tolerances. That answer would be easy to understand. However, that is obviously not the case since each individual drive model seems to have a consistent offset error. Why would the manufacturers deliberately "design" a certain fixed amount of offset error into a certain drive model and some other amount to another drive model?


Edit: typo

There are some answers somewhere in this thread.

Thanks. This link in the thread you linked seems to give some answers to the "why" question.